This invention relates to a method of and a system for positioning sheet elements in the introduction station of a machine for processing sheet material, and more particularly a system for positioning a sheet of paper, cardboard or similar material very accurately in the introduction station of the machine, just before the leading edge of said sheet is gripped by a transport system comprising a series of grippers mounted on a transverse bar connected to side chains subsequently feeding said sheet to the different subsequent processing stations of the machine, such as the printing, cutting or other stations.
In the positioning systems currently in use, the sheet is jogged on the feed table against front and lateral stops by means of feed members such as rollers or bands of elastic material descending from above on to the table, or a pair of rollers arranged above and below the table.
In the case of untreated sheets, the front and side stops used are disposed exactly with respect to the references of the subsequent stations. The sheet is then rapidly pushed against the stops by the feed members and then gripped by the gripper bar. The stops are then retracted and the gripper bar can pull the sheet to the following processing station and position it exactly with respect to the tools of the platen of that station.
Positioning is slightly more complicated when the sheet has previously undergone one or more processing operations, for example printing or scoring of fold lines, and the subsequent processing operations have to be carried out very accurately with reference to the results of the previous processing operations. The patent CH 676695 discloses a positioning system in which the stops are motor-driven so as to be able to control a variation in their position and wherein a printing mark or other distinctive sign of the previous processing operation can be logged by opto-electronic reading means in order to adjust the position of the stops, and hence of the sheet, directly in dependence on the mark or sign.
However, these stop type positioning systems mean that the advance of each sheet must be practically stopped for the entire duration of its jogging and adjustment operation, and this substantially limits the possible speed of movement of the sheets in the machine.
Also, the fact that a sheet is applied against a stop may result in the formation of an unsightly mark on the corresponding edge. The swelling of this mark may also interfere with the introduction of the sheet in critical circumstances.
The object of this invention is to provide a method of and a system for positioning sheet elements, such as sheets of paper, cardboard or similar material, in the introduction station of a processing machine, such method and systems being faster so as to allow the machine cycle to be increased while remaining very accurate, particularly in the case of sheets which have received a previous treatment. Preferably, the system should leave no mark on the sheet element. Nevertheless, the design of this system must remain relatively simple in order to increase its reliability, simplify maintenance, and reduce the production costs as far as possible.
These objectives are achieved by a method of positioning sheet elements in the introduction station of a processing machine comprising, starting from a shelf in a rear starting position, engaging means for fixing a sheet element on the shelf, then controlling actuators to advance the shelf forward and if necessary sideways and/or askew depending on the readings of opto-electronic means, said readings being effected at the start of the advance of the shelf so as to feed and finally stop the leading edge of the sheet element in the grippers of a drive bar in a predetermined position, releasing the fixing means, and then returning the shelf to the rear starting position.
Thus after the reading of the effective position of the sheet element with respect to the shelf by the opto-electronic means during a first phase of the shelf advance, the subsequent advance phase can be utilized to advantage to effect adjustments of the movement of the shelf with respect to its basic travel in order very dynamically to correct any longitudinal, lateral or skew errors of the sheet element. If the sheet element is untreated, the corrections can be advantageously carried out on the basis of logging its edges; if the sheet has previously been processed, the corrections are advantageously carried out on the basis of a reference mark of, or distinctive signs corresponding to, said treatment. The corrections thus made during sheet movement mean that it is no longer necessary to stop the sheet element uselessly, so that the possible rate of operation of the machine can be substantially increased.
The method of controlling the positioning system therefore comprises, starting from the rear starting position of the shelf, controlling the actuators to advance the shelf forward until the leading edge of the sheet element has been detected by the opto-electronic means, then controlling the corresponding actuator in order to move the shelf transversely until the side edge of the sheet element has been detected by the opto-electronic means.
Preferably, the control method comprises, starting from the shelf in the rear starting position, controlling the actuators to advance the shelf forward at a constant speed until the machine gripper bar arrives at the standby position for receiving a sheet element, then controlling the actuators to effect adjustments of the movement of the shelf with respect to its basic travel in order to correct any longitudinal, lateral or skew errors of the sheet element as determined by the readings of the opto-electronic means and finally to stop the leading edge of the sheet element in position in the grippers of the drive bar.
Thus by postponing the shelf correction movements to the end of its travel only when the gripper bar has arrived in the standby position, this obviates any adverse collision risk between the two elements which might be caused by unpredictable acceleration of the shelf.
The system for positioning sheet elements in the introduction station of a processing machine comprising a transport system made up of one or more gripper bars for driving the sheet element to the subsequent stations comprises:
a shelf provided with means for temporarily fixing the sheet element, having a notched leading edge complementary to the grippers of the drive bar driven by actuators on the one hand in reciprocating longitudinal translation from a rear starting position to a forward position at the level of a gripper bar in standby to receive a sheet element, and on the other hand in complementary longitudinal translation and/or in transverse translation and/or in rotation about a vertical axis,
opto-electronic means reading the front and/or side edge and/or a distinctive sign of a previous treatment of the sheet element disposed slightly forwardly of the starting position of the shelf,
electronic control means for controlling the actuators of the shelf in dependence on the machine cycle and the results of the readings of the opto-electronic means.
The pneumatic temporary fixing means for the sheet element on the shelf may comprise a network of apertures formed on the top surface of the shelf communicating with an internal chamber connected by an electromagnetic shut-off/opening valve to a negative pressure supply. However, pneumatic means of this kind prove to be relatively slow and complex to use.
According to a preferred embodiment, the temporary fixing means comprise a comb mounted pivotably above, near and parallel to the front notched edge of the shelf, the teeth of the comb being oriented forward so that each of them forms with each projection of the shelf separating a notch a gripper for gripping the leading edge of the sheet element, and means for controlling the angular position of the comb.
Advantageously, the means for controlling the angular position of the comb comprise on the one hand one or more springs disposed between the shelf and the rear bar connecting the teeth of the comb so that, at rest, said spring raises the rear bar and holds the comb closed, i.e. with the teeth bearing on their shelf projection, and on the other hand, connected to the station, one or more subjacent control means having a vertical movement, such that the end of a lever actuated for rotation or the output rod of a pneumatic or electropneumatic jack, and each connected by a telescopic pendular link to the rear bar of the comb, so that in the bottom position it can bear on and lower the rear bar of the comb and thus hold the comb open during a movement of the shelf.
Thus when the pendular telescopic link is in the top position with its rod sliding freely on the advance of the shelf, the spring ensures effective closure of the comb, hence firm holding of the sheet element during its feed and its position corrections. On the other hand, when the telescopic link is in the bottom position with its rod in abutment on withdrawal of the shelf on its return to its rear starting position, it opens the comb and holds it open in this way despite the movement of said shelf as a result of the pendular arrangement of said link.
Thus the sheet element is held along its leading edge uniformly and with a practically constant pressure in order to obviate the formation of any adverse mark. Also, the fact that said leading edge is held flat on the shelf projections enables said sheet element edge to be inserted without any risk of snagging in the open grippers of the machine drive bar.
According to a preferred embodiment, the longitudinal translation is effected by a precision actuator which provides the movement and longitudinal position correction.
According to one advantageous embodiment, the shelf is mounted on each side on a lateral vertical pivot respectively connected to a carriage moving along a longitudinal guide by means of a linear actuator supported by the base cross-member.
In this case, when the actuators advance in synchronism, the shelf simply performs a purely longitudinal complementary translatory movement in the forward direction. On the other hand, the introduction of a slight offset between the movements of each of the lateral actuators enables the shelf to be voluntarily put askew by rotation about a vertical axis to take up any initial skewing of the sheet element.
According to another advantageous embodiment, the shelf is mounted at its centre on a central vertical pivot and on each side on a vertical lateral pivot, the central pivot being connected to a carriage sliding in a longitudinal guide adapted to be moved transversely by a first linear actuator, each side pivot being connected to a carriage sliding in a transverse guide adapted to be moved longitudinally respectively by a second and third linear actuator, the first, second and third actuators being supported on the base cross-member.
As before, when the second and third actuators advance in synchronism, the shelf simply effects a purely longitudinal forward translatory movement, the carriage of the central pivot simply following the movement within its longitudinal guide. Also, the introduction of a slight offset between the movements of the second and third actuators enables skewing to be voluntarily induced in the shelf to take up any corresponding initial error of the sheet element. On the other hand, the use of the first actuator results in a transverse movement of the shelf, the carriages of the lateral pivots then only following that movement within their transverse guide.
As a result of this symmetrical kinematics with respect to the central longitudinal vertical plane, it is a simple matter to transmit to the shelf any longitudinal, transverse or pivoting correction on itself, by separate control of just three linear actuators.
In this latter positioning system according to the invention, the position control of the first actuator is carried out with respect to a reference established in dependence on the transverse position error of the sheet element as read by the opto-electronic means. The second and third actuators are in turn controlled in position with respect to a correction reference for longitudinal errors and/or skew errors as also read by the opto-electronic means. With regard to the shelf actuator, its position is controlled by a reducing reference with respect to the machine cycle, which is usually denoted by the main motor output shaft angle, said reducing reference being representative of the basic front to rear movement for feeding the sheet element.
The linear actuator used may be a carriage sliding along a guide and the transverse tapping of which is engaged on an endless-thread rod, one of the ends of which is driven in rotation by an electric motor. Another possibility is a rack mounted to slide freely on bearings and with the teeth meshing with a pinion driven by an electric motor. However, these actuators make use of intermediate mechanical links which transform the rotary movement of the output shaft of an electric motor into a translatory movement, and these connections always have a relative inertia and some play in operation.
According to a preferred embodiment, the linear actuators are linear electric motors the rotor of which is in the form of a bar movable in translation and provided with a series of permanent magnets moving with respect to one or more loops of a stator supplied with electrical current, said bar directly supporting either the pivot support or the transverse guide within which the pivot carriage slides.
By means of such linear electric motors there is very dynamic direct action on the part being moved, whether the pivot support or an intermediate guide. In particular, in the case of any risk of collision between the shelf and the gripper bar all that is necessary is to reverse the polarity with which the stator loops are connected in order to be able to instantly stop and withdraw the shelf.
In the case of untreated sheet elements, the opto-electronic means for detecting the edges of the element may comprise just one pair of front photoelectric cells and one lateral photoelectric cell.
According to an advantageous alternative, the opto-electronic means for detecting the side edges of the sheet element may be an array of photoelectric cells or CCD strip, enabling a transverse movement to be transmitted to the shelf in order specially to log the side edge.
In the case of sheet elements which have previously undergone one or more processing operations, the opto-electronic means for detecting the or each mark or other distinctive signs of the previous processing operation may be a CCD array or some other camera capturing an image of the sheet element on the movement of the shelf. Electronic means are known for processing the image to enable the marks and their forms to be recorded in order to deduce the longitudinal and lateral positioning errors therefrom.